For the values I assume I need something like:

NameFieldValue = GetParamValue(parameter.NameFieldValue) ?? "Some Value";

and then:

@RenderTextField(Page.FormVariables.Name, Page.FormVariables.NameFieldName, Page.FormVariables.NameFieldValue, Page.FormVariables.NameFieldError, "text")

The form renders and the error messages display as they should but I get an Error loading macroscript when I try to send the form. Here is the script:

@{
    Page.FormVariables = new FormVariables(Parameter, Model);
    Page.Errors = new List<string>();
    Page.Model = Model;
}


    @(HttpContext.Current.Request["form-posted"] != "1" ? RenderContactForm() : TrySendForm())


@helper RenderContactForm()
{
   
    <form action="#newsletterform" method="post">
        <fieldset>
        <legend>@Dictionary["Newsletter Heading"]</legend>
            <input type="hidden" name="form-posted" value="1" />

            @RenderTextField(Page.FormVariables.Name, Page.FormVariables.NameFieldName, Page.FormVariables.NameFieldError, "text")
            @RenderTextField(Page.FormVariables.Email, Page.FormVariables.EmailFieldName, Page.FormVariables.EmailFieldError, "text email")

           
                <input class="submit" type="submit" value=@Dictionary["Submit Button"] id="newsletter"  />

                @if (Page.Errors.Count > 0 && Page.Errors.Contains("Other"))
                {
                    <div class="note">@Page.FormVariables.ErrorSendingForm</div>
                }
           
        </fieldset>
    </form>
}

@helper RenderTextField(string fieldLabel, string fieldName, string errorText, string className)
{
   
        <label class="label" for="@fieldName">@fieldLabel</label>
        if (Page.Errors.Contains(fieldName))
        {
            <span class="error">@errorText</span>
        }
        <input type="text" id="@fieldName" name="@fieldName" class="@className @(Page.Errors.Contains(fieldName) ? "error" : "")" value="@HttpContext.Current.Request[fieldName]"/>
       
   
}

   
@helper TrySendForm()
    {
    if (Request.Url.AbsoluteUri.Contains("form-posted") == false)
    {
        Page.Errors = GetFormErrors();
        if (Page.Errors.Count == 0)
        {
            Page.Errors.AddRange(TrySendMail(Page, GetMailVariables(Page)));

            if (Page.Errors.Count == 0)
            {
                Response.Redirect(Page.FormVariables.RedirectUrl);
            }
        }
        else
        {
            @RenderContactForm()
        }
    }

}
@functions
{
    private static List<string> GetFormErrors()
    {
        var errorFields = new List<string>();
        foreach (string key in HttpContext.Current.Request.Form)
        {
            if (key.EndsWith("-req") && HttpContext.Current.Request.Form[key] == "")
            {
                errorFields.Add(key);
            }
            if (key.EndsWith("-req-mail") && (HttpContext.Current.Request.Form[key] == "" || System.Text.RegularExpressions.Regex.IsMatch(HttpContext.Current.Request.Form[key], @"[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-zA-Z0-9](?:[a-zA-Z0-9-]*[a-z0-9])?\.)+[a-zA-Z0-9](?:[a-zA-Z0-9-]*[a-zA-Z0-9])?") == false))
            {
                errorFields.Add(key);
            }
        }
        return errorFields;
    }
   
    private static MailVariables GetMailVariables(dynamic page)
    {
        var mailVariables = new MailVariables
                                {
                                    Content = GetMailContent(page, page.FormVariables.MailIntroText),
                                    From = page.FormVariables.MailFrom,
                                    // To make sure your mail isn't marked as spam the from address
                                    // is always the configured "from" address, make sure the domain
                                    // corresponds with your SMTP server configuration
                                   
                                    FromName = HttpUtility.HtmlEncode(HttpContext.Current.Request[page.FormVariables.NameFieldName]),
                                    ReplyTo = HttpUtility.HtmlEncode(HttpContext.Current.Request[page.FormVariables.EmailFieldName]),
                                    Subject = page.FormVariables.MailSubject,
                                    To = page.FormVariables.MailFrom,
                                    ToName = page.FormVariables.MailFromName,
                                    EnableSsl = page.FormVariables.EnableSsl
                                };

        return mailVariables;
    }
   
   
    private static string GetMailContent(dynamic page, string mailIntroText)
    {
        var name = HttpUtility.HtmlEncode(HttpContext.Current.Request[page.FormVariables.NameFieldName]);
        var email = HttpUtility.HtmlEncode(HttpContext.Current.Request[page.FormVariables.EmailFieldName]).Replace("\n", "<br />");

        var mailContent = string.Format(
                                 "{0}<br /><br /><table><tr><td valign=\"top\"><p><strong>{1}</strong></p></td><td valign=\"top\"><p>{2}</p></td></tr><tr><td valign=\"top\"><p><strong>{3}</strong></p></td><td valign=\"top\"><p>{4}</p></td></tr><tr><td valign=\"top\"><p><strong>{5}</strong></p></td><td valign=\"top\"><p>{6}</p></td></tr></table>",
                                 SurroundWithArial(mailIntroText),
                                 SurroundWithArial(page.FormVariables.Name),
                                 SurroundWithArial(name),
                                 SurroundWithArial(page.FormVariables.Email),
                                 SurroundWithArial(email)
                    );


        if (page.Model.MailTemplate.GetType() != typeof(umbraco.MacroEngines.DynamicNull))
        {
            var mailTemplateProperty = page.Model.MailTemplate;
            var mailTemplate = mailTemplateProperty.GetType() == typeof(umbraco.MacroEngines.DynamicXml)
                                   ? mailTemplateProperty.ToXml().ToString().Trim()
                                   : mailTemplateProperty.ToString();

            if (mailTemplate != "")
            {
                mailTemplate = mailTemplate.Replace("%%INTROTEXT%%", mailIntroText);
                mailTemplate = mailTemplate.Replace("%%NAMELABEL%%", page.FormVariables.Name);
                mailTemplate = mailTemplate.Replace("%%NAME%%", name);
                mailTemplate = mailTemplate.Replace("%%EMAILLABEL%%", page.FormVariables.Email);
                mailTemplate = mailTemplate.Replace("%%EMAIL%%", email);

                mailContent = mailTemplate;
            }
        }

        return mailContent;
    }
   
    private static string SurroundWithArial(string input)
    {
        return string.Format("<font face=\"Arial\" size=\"3\">{0}</font>", input);
    }
   
    private static IEnumerable<string> TrySendMail(dynamic page, MailVariables mailVariables)
    {
        var mailSent = SendMail(mailVariables);
        if (mailSent == false)
        {
            page.Errors.Add("Other");
        }

        return page.Errors;
    }

    public static bool SendMail(MailVariables mailVariables)
    {
        try
        {
            var mailMsg = new System.Net.Mail.MailMessage
            {
                From = new System.Net.Mail.MailAddress(HttpUtility.HtmlEncode(mailVariables.From), HttpUtility.HtmlEncode(mailVariables.FromName)),
                Subject = mailVariables.Subject,
                Body = mailVariables.Content,
                IsBodyHtml = true
            };

            mailMsg.To.Add(new System.Net.Mail.MailAddress(HttpUtility.HtmlEncode(mailVariables.To), HttpUtility.HtmlEncode(mailVariables.ToName)));
            mailMsg.Bcc.Add(new System.Net.Mail.MailAddress(HttpUtility.HtmlEncode(mailVariables.From)));
            mailMsg.ReplyToList.Add(new System.Net.Mail.MailAddress(mailVariables.ReplyTo));

            var smtpClient = new System.Net.Mail.SmtpClient { EnableSsl = mailVariables.EnableSsl };

            smtpClient.Send(mailMsg);
            return true;
        }
        catch (Exception ex)
        {
            umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error, -1, string.Format("Error creating or sending mail, check if there is a mailFrom property on your document and that it has a value, or specify a MailFrom parameter on the macro call exception: {0}", ex.InnerException));
        }

        return false;
    }

    public class FormVariables
    {
        public FormVariables(dynamic parameter, dynamic model)
        {
            var modelMailFrom = model.MailFrom.GetType() != typeof(umbraco.MacroEngines.DynamicNull) ? model.MailFrom : "";
            var modelMailFromName = model.MailFromName.GetType() != typeof(umbraco.MacroEngines.DynamicNull) ? model.MailFromName : "";

            MailFrom = GetParamValue(parameter.MailFrom) ?? GetParamValue(modelMailFrom);
            MailFromName = GetParamValue(parameter.MailFromName) ?? GetParamValue(modelMailFromName);

            Name = GetParamValue(parameter.FormLabelName) ?? "Name";
            Email = GetParamValue(parameter.FormLabelEmail) ?? "E-mail";

            ErrorsInForm = GetParamValue(parameter.FormValidationError) ?? "Please check the highlighted fields for errors";
            ErrorsendingForm = GetParamValue(parameter.FormGenericError) ?? "There was a technical error sending the form, please try again or contact us in an alternative way.";

            MailSubject = GetParamValue(parameter.MailSubject) ?? "Newsletter mail";
            MailIntroText = GetParamValue(parameter.MailIntroText) ?? "Newsletter request from NEWSBPP:";

            FormSentConfirmation = GetParamValue(parameter.FormSentConfirmation) ?? "Thank you, we will get back to you soon.";

            RedirectUrl = GetParamValue(parameter.RedirectUrl) ?? model.Url + "?form-posted=1";

            bool enableSsl;
            bool.TryParse(parameter.EnableSsl, out enableSsl);
            EnableSsl = enableSsl;

            NameFieldName = "name-req";
            NameFieldError = GetParamValue(parameter.NameFieldError) ?? "Name required";
            EmailFieldName = "email-req-mail";
            EmailFieldError = GetParamValue(parameter.EmailFieldError) ?? "Invalid e-mail address";

        }

        public string MailFrom { get; set; }
        public string MailFromName { get; set; }

        public string Name { get; set; }
        public string Email { get; set; }

        public string ErrorsInForm { get; set; }
        public string ErrorsendingForm { get; set; }

        public string MailSubject { get; set; }
        public string MailIntroText { get; set; }

        public string MailSubjectCopyToSender { get; set; }
        public string MailIntroTextCopyToSender { get; set; }

        public string FormSentConfirmation { get; set; }

        public string RedirectUrl { get; set; }
        public bool EnableSsl { get; set; }

        public string NameFieldName { get; set; }
        public string NameFieldError { get; set; }
        public string EmailFieldName { get; set; }
        public string EmailFieldError { get; set; }

        public static string GetParamValue(dynamic input)
        {
            if (input != null && input.StartsWith("[!") && input.EndsWith("]"))
            {
                var dictKey = input.Substring(2, input.LastIndexOf("]") - 2);
                input = library.GetDictionaryItem(dictKey);
            }

            return input;
        }
    }

    public class MailVariables
    {
        public string Content { get; set; }
        public string Subject { get; set; }
        public string To { get; set; }
        public string ToName { get; set; }
        public string From { get; set; }
        public string FromName { get; set; }
        public string ReplyTo { get; set; }
        public bool EnableSsl { get; set; }
    }
}

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